Solenoid-operated valve

ABSTRACT

Disclosed is a solenoid-operated valve which hardly brings a plunger into a lock and is capable of improving its responsivety. In the solenoid-operated valve, the plunger slidably guided in an inner bore formed in a yoke and a core aligned axially is moved by energizing an electromagnetic coil, and a spool in a valve section is moved through a rod portion which protrudes from an end of the plunger to pass through a center hole of the core. An electromagnetic section fluid chamber defined by a forward end surface of the plunger and the inner bore of the core communicates with an intermediate fluid chamber defined between the core and a valve sleeve, through a clearance between the center hole and rod portion. The spool is provided at one end thereof with a land portion whose end surface defines a part of the intermediate fluid chamber. The diameter of the land portion is chosen to be the same as the diameter of the plunger, so that the sum in volume of the electromagnetic section fluid chamber and the intermediate fluid chamber can be kept invariable regardless of movement of the plunger. A supply/drain passage is further provided to permit the oil around the solenoid-operated valve to be charged into the rear end fluid chamber or discharged therefrom upon movement of the plunger.

[0001] This application is based on and claims priority under 35 U.S.C.sctn. 119 with respect to Japanese Application No. 2003-35238 filed onFeb. 13, 2003, the entire content of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a solenoid-operated valve of thetype that a spool of a valve section is operated upon movement in theaxial direction of a plunger of an electromagnetic drive section and inparticular, to a solenoid-operated valve which is suitable for useinside of an oil pan of an electronic controlled automatic transmissionfor a motor vehicle.

[0004] 2. Discussion of the Related Art

[0005] Heretofore, as solenoid-operated valves of this type, there hasbeen known one described in the item of “Prior Art” of a Japaneseunexamined, published patent application No. 1-242884 (1989-242884). Inthe known solenoid-operated valve, a first solenoid housing (core) and asecond solenoid housing (yoke) are arranged serially in axial alignment,and a plunger is slidably guided in a guide bore formed in the solenoidhousings. By magnetizing the solenoid housings with a solenoid, theplunger is axially moved against a spring, so that a spool in a spool orvalve housing attached to the first solenoid housing (core) is operated.Since the space, defined between an end surface of the plunger and acover, in the guide bore formed in the second solenoid housing (yoke) isto vary its volume as the plunger moves, it is in communication with theexternal of the solenoid-operated valve through a supply/drain passageformed at, e.g., the center of the plunger to pass therethrough. In thistechnology, the plunger is slidably guided in the guide bore, and thismay give rise to brining the plunger into a lock when foreign mattergets in-between the internal surface of the guide bore and the externalsurface of the plunger.

[0006] To obviate this problem, in a known solenoid-operated valvedescribed in the body of the aforementioned Japanese application, aplunger is constituted by a movable member made of a magnetic materialand a rod. The movable member with a press-fitting hole at the centerthereof is inserted into a guide bore formed in solenoid housings to bemovable back and forth, and the rod is press-fit into the press-fittinghole of the movable member to be protruded from the movable membertoward a spool. One end of the rod is press-fit into a fitting holeformed on one end of the spool so that the movable member is supportedby the spool without being contacted with the guide bore.

[0007] However, in the latter mentioned technology, it is unavoidable toincrease the space between the external surface of the plunger made of amagnetic material and the internal surface (i.e., the guide bore) of thesolenoid housings. This causes the magnetic resistance in the magneticcircuit to increase thereby to weaken the magnetization of the solenoidhousings and the movable member. Thus, the magnetic attracting forceexerted on the plunger is weakened, whereby the operating characteristicof the solenoid valve becomes unstable, or whereby the solenoid has tobe enlarged to obtain the same operating characteristic.

[0008] A technology shown in FIG. 3 may be conceived of as one which isable to solve the aforementioned drawback. The solenoid-operated valveshown in FIG. 3 is composed of an electromagnetic drive section 1 and avalve section 5. In the electromagnetic drive section 1, a yoke 2 a anda core 2 b each made of a magnetic material are arranged serially inaxial alignment through an air gap (i.e., non-magnetic portion) 2 d, andthe axial opposite ends of the yoke 2 a and the core 2 b aremagnetically connected with each other through a cover 2 c which coversthe yoke 2 a and the core 2 b thereby to constitute a stator 2. Aplunger 3 is slidably guided in a bore 2 e which is formed in the stator2 over the yoke 2 a and the core 2 b, and an electromagnetic coil 4 isprovided between the yoke 2 a and the core 2 b inside the cover 2 c.Further, the valve section 5 is constituted by inserting a spool 7slidably in a valve hole of a valve sleeve 6 attached to the stator 2.The spool 7 is urged toward the plunger 3 by means of a spring 9 ainterposed between itself and a plug member 9 secured to a forward endportion of the valve sleeve 6 and is brought into abutting engagementwith the plunger 3 at a rod portion 7 c thereof protruding from a rearend portion thereof. Thus, in the inoperative state, the rear end of theplunger 3 is kept in abutting contact with an inner bottom surface ofthe cover 2 c through a washer, as depicted at the upper half in FIG. 3.When electric current is applied to the electromagnetic coil 4, on thecontrary, the stator 2 is magnetized in proportion to the magnitude ofthe electric current applied thereto. Thus, the plunger 3 is attractedtoward the core 2 b against the spring 9 a thereby to operate the valvesection 5 through the movement of the spool 7, as depicted at the lowerhalf in FIG. 3.

[0009] This solenoid-operated valve is designed for use, e.g., with thevalve sleeve 6 being inserted fluid-tightly into a fitting hole of avalve body (not shown) which is provided inside an oil pan of anautomatic transmission. The opening degrees of a supply port 6 a and adrain port 6 c of a valve sleeve 6 at both sides of a control port 6 bare increased or decreased reversely with each other in proportion tothe moving amount of the spool 7 having two large-diameter land portions7 a, whereby the oil output from the control port 6 b is controlled inpressure as well as in volume. An annular groove which is formed insidethe valve sleeve 6 between one of the large-diameter land portions 7 aand a small-diameter land portion 7 b is isolated from the externalthereby to define a feedback chamber 6 d, to which the control pressurein the control port 6 b is applied through a cutout 6 e formed at a partof the external surface of the valve sleeve 6 and also through acommunication hole 6 f.

[0010] A rear end fluid chamber (E) is defined between the rear endsurface of the plunger 3 and the inner bottom surface of the cover 2 cin the bore 2 e formed in the yoke 2 a. An electromagnetic section fluidchamber (F) is formed between the bore 2 e of the core 2 b and theforward end surface of the plunger 3. The rear end fluid chamber (E)communicates with the electromagnetic section fluid chamber (F) throughcommunication grooves 3 a formed over the entire length of the plunger3. An intermediate fluid chamber (H) is formed between the core 2 b andthe valve sleeve 6 and communicates with the electromagnetic sectionfluid chamber (F) through a clearance (G) between a center hole of thecore 2 b and a rod portion 7 c of the spool 7. Further, the intermediatefluid chamber (H) further communicates with the external of thesolenoid-operated valve through a communication passage 8. That is, thecommunication grooves 3 a, the electromagnetic section fluid chamber(F), the clearance (G), the intermediate fluid chamber (H) and thecommunication passage 8 constitute a supply/drain passage, through whichthe rear end fluid chamber (E) is in fluid communication with theexternal of the solenoid-operated valve. The communication passage 8takes the form of a labyrinth which is composed of an annular groove 8 aformed coaxially with the intermediate fluid chamber (H) and cutouts 8b, 8 c which make the annular groove 8 a communicate with theintermediate fluid chamber (H) as well as with the external of thesolenoid-operated valve. The rear end fluid chamber (E) varies itsvolume in proportion to the movement of the plunger 3, and in the casewhere the solenoid-operated valve is provided inside the oil pan, suchvariation in volume causes the oil around the valve sleeve 6 to becharged into the rear end fluid chamber (E) through the supply/drainpassage or to be discharged therefrom.

[0011] In the technology shown in FIG. 3, since the plunger 3 made of amagnetic material is received slidably in the bore 2 e formed in theyoke 2 a and the core 2 b and since the clearance therebetween is small,it does not take place that the magnetic resistance in the magneticcircuit constituted by these members increases to weaken themagnetization of the stator 2 and the plunger 3, and thus it is nolonger required to make the magnetic coil 4 large for the same operatingcharacteristic. Further, the surrounding oil around thesolenoid-operated valve is charged into or discharged from theelectromagnetic section fluid chamber (F) through the communicationpassage 8, the intermediate fluid chamber (H) and the clearance (G).However, since the communication passage 8 of a labyrinth shape composedof the annular groove 8 a and the cutouts 8 b, 8 c is long and extendsto move up and down in the case that the solenoid operated valve is usedwith the axis thereof extending horizontally or being inclined slightlywith respect to a horizontal axis, the foreign matter such as ironpowder which floats in the surrounding oil subsides or deposits to beseparated from the oil while being moved back and forth through theannular groove 8. In addition to this, since the intermediate fluidchamber (H) has a substantial volume, it does not occur that the foreignmatter easily comes into the electromagnetic section fluid chamber (F).It is to be noted that the technology shown in FIG. 3 is presented herefor the purpose of comparison and does not constitute any known art. Ofcourse, there cannot be found any literature which shows and describesthe construction shown in FIG. 3.

[0012] However, in the technology shown in FIG. 3, the oil which passesthrough the clearance (G) between the center hole of the core 2 b andthe rod portion 7 c of the spool 7 flows toward the left when theplunger 3 is moved toward the right against the spring 9 a and towardthe right when the plunger 3 is moved toward the left. That is, themoving direction of the rod portion 7 c and the flowing direction of theoil passing through the clearance (G) are opposite to each other. Sincethe oil flow impedes the movement of the spool 7 due to the viscosityresistance, there is raised a problem that the responsivety of thesolenoid-operated valve is degraded. This problem emerges remarkablywhen the viscosity of the oil is large at a low temperature.

[0013] In addition, the feedback chamber 6 d to which the controlpressure from the control port 6 b is applied and the intermediate fluidchamber (H) whose inside pressure is zero by being in communication withthe external through the communication passage 8 are separated by afitting portion between the valve hole of the valve sleeve 6 and thesmall-diameter portion 7 b of the spool 7, and the fitting portion has asmall clearance for allowing the sliding movement of the spool 7. Thus,minute foreign matter floating in the oil supplied from the supply port6 a is led to the feedback chamber 6 d through the cutout 6 e and thecommunication hole 6 f and then, leaks from the feedback chamber 6 dthrough the small clearance into the intermediate fluid chamber (H)thereby to increase the amount of the foreign matter in the intermediatefluid chamber (H). Therefore, it may be liable that at an earlier stagethan as expected, the foreign matter gets into the clearance between theinternal surface of the hole 2 e and the external surface of the plunger3 to bring the plunger 3 into a lock.

SUMMARY OF THE INVENTION

[0014] Accordingly, it is a primary object of the present invention toprovide an improved solenoid-operated valve which is capable ofpreventing a plunger from being locked and also capable of enhancing theresponsivety thereof.

[0015] Briefly, according to the present invention, there is provided asolenoid-operated valve which comprises a stator having a yoke and acore arranged serially in axial alignment with each other, a plungerreceived in the stator to be slidably guided in an inner bore formed inat least one of the yoke and the core and urged by a spring in onedirection, an electromagnetic coil for energizing the stator to move theplunger axially against the spring, a valve sleeve attached to an outerend portion of the core, and a spool received slidably in a valve holeof the valve sleeve and connected to the plunger through a rod portionthereof passing through a center hole of the core to be movable togetherwith the plunger. An electromagnetic section fluid chamber is defined bya forward end of the plunger at the side of the valve sleeve and theinner bore of the core, and an intermediate fluid chamber is formedbetween the core and the valve sleeve and defined partly by an endsurface of a land portion which is formed on the spool at the side ofthe core. A clearance is provided between the center hole and the rodportion for making the electromagnetic section fluid chamber and theintermediate fluid chamber communicate with each other. A rear end fluidchamber is formed between a rear end surface of the plunger and theinner bore of the yoke and is variable in its volume with the movementof the plunger. Further, the diameter of the plunger is chosen to be thesame as that of the land portion, and the rear end fluid chambercommunicates with the external of the solenoid-operated valve through asupply/drain passage.

[0016] With this configuration, since the diameter of the plunger ismade the same as the diameter of the land portion, the sum in volume ofthe electromagnetic section fluid chamber and the intermediate fluidchamber which are in communication with each other through the clearancebetween the center hole and the rod portion is kept invariable orconstant regardless of the movement of the plunger. The supply/drainpassage permits the oil outside to be charged into or discharged fromthe rear end fluid chamber whose volume varies with the movement of theplunger. Consequently, it becomes unnecessary to provide anycommunication passage which would otherwise become necessary for makingthe intermediate fluid chamber communicate with the external and anycommunication groove (or communication hole, hereafter referred tosimply as “communication groove) which would also otherwise becomenecessary for communication between the rear end fluid chamber and theelectromagnetic section fluid chamber. In addition, the direction inwhich the oil flows through the clearance between the center hole andthe rod portion upon movement of the plunger coincides with the movingdirection of the rod portion. Thus, the viscosity of the oil flowingthrough the center hole of the core serves to aid or expedite themovement of the rod portion which is moved together with the spool, sothat the responsivety of the solenoid-operated valve can be enhanced.Further, there can be omitted any communication passage for making theintermediate fluid chamber communicate with the external and anycommunication groove formed on the plunger for communication between therear end fluid chamber and the electromagnetic section fluid chamber,the solenoid-operated valve can be simplified in construction. Inparticular, since the omission of such communication groove contributesto increasing the magnetic attraction force exerted on the plunger, thesolenoid-operated valve can be downsized.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0017] The foregoing and other objects and many of the attendantadvantages of the present invention may readily be appreciated as thesame becomes better understood by reference to a preferred embodiment ofthe present invention when considered in connection with theaccompanying drawings, wherein like reference numerals designate thesame or corresponding parts throughout several views, and in which:

[0018]FIG. 1 is a longitudinal sectional view of a solenoid-operatedvalve in the first embodiment according to the present invention;

[0019]FIG. 2 is a sectional view of the solenoid-operated valve takenalong the line A-A in FIG. 1; and

[0020]FIG. 3 is a longitudinal sectional view of a solenoid-operatedvalve which is hypothetically conceived of under the technologicalbackground prior to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Hereinafter, a solenoid-operated valve in the first embodimentaccording to the present invention will be described with reference toFIGS. 1 and 2. The solenoid-operated valve in this particular embodimentis composed of an electromagnetic drive section 10 and a valve section20 which are provided in axial alignment with each other.

[0022] As shown in FIG. 1, the electromagnetic drive section 10 iscomposed primarily of a stator 11, a plunger 16 and an electromagneticcoil 17. The stator 11 includes a yoke 12 and a core 13 which are eachmade of a magnetic material and which are held in axial alignmentthrough a pipe 15 made of a non-magnetic material, with an air gap(i.e., non-magnetic portion) therebetween. The stator 11 furtherincludes a cover 14 of a magnetic material which covers the yoke 12 andthe core 13 to magnetically connect the axial opposite ends thereof. Aninner bore 11 a of a single diameter is formed in the yoke 12 and thecore 13 over the same. The yoke 12 is provided with a small-diameterportion inserted into an electromagnetic coil 17 and a large-diameterportion at the rear end thereof. The core 13 is provided with asmall-diameter portion inserted into the electromagnetic coil 17 and aflange portion at a forward end portion opposite to the yoke 12. Thecore 13 is formed therein with a center hole 13 a in axial alignmentwith the inner bore 11 a. The cover 14 takes a cylindrical shape with abottom (i.e., cup shape) whose inner surface is kept in abutting contactwith the rear end surface of the yoke 12. As described later in detail,the opening end portion of the cover 14 is caulked thereby to hold theflange portion of the core 13 and the rear end surface of a valve sleeve21 of the valve section 20 in abutting engagement with each other.

[0023] The plunger 16 is made of a magnetic material in its entirety andis guided and supported slidably in the inner bore 11 a of the stator11. The plunger 16 is movable between an advanced position (shown at thelower half in FIG. 1) where its forward end surface 16 a at the side ofthe valve section 20 abuts on an inner end surface of the inner bore 11formed in the core 13 through a washer 16 c, and a retracted position(shown at the upper half in FIG. 1) where its rear end surface 16 babuts on the inner bottom surface of the cover 14. In the inner bore 11,an electromagnetic section fluid chamber (B) is defined between theforward end surface 16 a of the plunger 16 and the inner bore 11 a ofthe core 13, while a rear end fluid chamber (A) is defined between theinner bottom surface of the cover 14 and the inner bore 11 a of the yoke12. The inner bore 11 a of the yoke 12 is formed with a shallow cutoutgroove 12 a at a position corresponding to the rear end fluid chamber(A).

[0024] As best shown in FIG. 2, the rear end surface of thelarge-diameter portion of the yoke 12 is formed with an annular groove18 a at a radially mid position coaxially of the inner bore 11 a and isalso formed with a chamfer 18 b of the same depth as the annular groove18 a at the outer circumferential portion thereof. The cutout groove 12a formed at the inner bore 11 a is in communication with the annulargroove 18 a through an inside cutout 18 c, while the annular groove 18 ais in communication with the chamfer 18 b through an outside cutout 18d. These inside and outside cutouts 18 c, 18 d are formed respectivelyat diametrically opposite positions (different 180 degrees in angularphase) on the rear end surface of the yoke 12. A hole 14 a is formed inthe cover 14 at a position which has the same angular phase as theinside cutout 18 c and which is close to the inner bottom surface of thecover 14. The cover 14 covers the yoke 12 with its inner bottom surfaceabutting on the rear end surface of the yoke 12 which has the annulargroove 18 a, the chamfer 18 b and the inside and outside cutouts 18 c,18 d formed thereon, so that the annular groove 18 a, the chamfer 18 band the inside and outside cutouts 18 c, 18 d are walled up thereby toform a supply/drain passage 18 of a labyrinth shape. Therefore, the rearend fluid chamber (A) is in communication with the external of theelectromagnetic drive section 10 through the cutout groove 12 a, thelabyrinth supply/drain passage 18 and the hole 14 a.

[0025] Referring back to FIG. 1, the vale section 20 is composed of thevalve sleeve 21 and a spool 26 slidably received in a valve hole 22which is formed coaxially in the valve sleeve 21. The valve sleeve 21 issecured to the stator 11 (i.e. to the core 13) in axial alignmenttherewith by caulking the opening end portion of the cover 14 with itsflange portion being in abutting contact with the flange portion at theforward end portion of the core 13. The spool 26 is urged toward theelectromagnetic drive section 10 by means of a spring 29, which isinterposed between itself and a plug member 28 screwed into a forwardend of the valve sleeve 21. A rod portion 26 a which is formed toprotrude from a rear end of the spool 26 extends passing through thecenter hole 13 a of the core 13 and abuts on the forward end surface 16a of the plunger 16. Thus, in the inoperative state, the plunger 16 iskept at the aforementioned retracted position where the rear end surface16 b thereof abuts on the inner bottom surface of the cover 14. Anintermediate fluid chamber (D) formed at the abutting portion betweenthe core 13 and the valve sleeve 21 communicates with theelectromagnetic section fluid chamber (B) through an annular clearance(C) which is formed between the center hole 13 a of the core 13 and therod portion 26 a of the spool 26.

[0026] The valve hole 22 of the valve sleeve 21 has a large-diameterportion 22 a at the side of the spring 29 and a small-diameter portion22 b opening to the intermediate fluid chamber (D) at the side of thecore 13. The spool 26 has two large-diameter land portions 27 a fit inthe large-diameter portion 22 a and a small-diameter land portion 27 bfit in the small-diameter portion 22 b. The diameter (d1) of thesmall-diameter land portion 27 b is chosen to be the same as that (d2)of the plunger 16, and the rear end surface of the land portion 27 bdefines a part of the intermediate fluid chamber (D).

[0027] A control port 23 b opens at the axial mid position of the valvehole 22 of the valve sleeve 21, and a supply port 23 a and a drain port23 c are formed respectively at opposite sides of the control port 23 b.The opening degree in communication of the control port 23 b with thesupply port 23 a and the opening degree in communication of the controlport 23 with the drain port 23 c are increased or decreased reverselywith each other in proportion to the moving amount of the spool 26having the two large-diameter land portions 27 a, whereby the oil outputfrom the control port 23 b can be controlled in pressure as well as involume. At the internal surface of the valve hole 22 of the valve sleeve21, an annular groove is formed between the large diameter portion 22 aand the small-diameter portion 22 b thereby to define a feedback chamber24 which is isolated from the external. This feedback chamber 24 is incommunication with the control port 23 b through a communication hole 24b and a cutout portion 24 a which is formed at a part of the externalsurface of the valve sleeve 21. This cutout portion 24 a defines aclosed space when the solenoid-operated valve is put in use with thevalve sleeve 21 being fit in a valve sleeve receiving bore (not shown).An annular groove 25 is further formed at an axial mid position of thesmall-diameter portion 22 b of the valve hole 22, namely, between thefeedback chamber 24 and the intermediate fluid chamber (D). The insideof the annular groove 25 is in communication with another drain port 23c, and thus, the mid portion of the small-diameter land portion 27 b isin communication with the drain port 23 c.

[0028] When electric current is applied to the electromagnetic coil 17,the stator 11 is magnetized in proportion to the magnitude of theelectric current applied thereto thereby to make the plunger 16attracted toward the core 13, and the valve section 20 is operated tomove the spool 26 against the spring 29, as depicted at the lower halfin FIG. 1. With movement of the spool 26, the rear end fluid chamber (A)varies in volume. Therefore, the oil around the solenoid-operated valveis charged into the rear end fluid chamber (A) through the hole 14 a,the labyrinth supply/drain passage 18 and the cutout 12 a. Conversely,when the electric current applied to the electromagnetic coil 17 isdiminished in magnitude, the plunger 16 is retracted by means of thespring 29, and the oil in the rear end fluid chamber (A) is dischargedtherefrom back through the cutout 12 a, the labyrinth supply/drainpassage 18 and the hole 14 a.

[0029] In the foregoing embodiment, the diameter (d1) of the landportion 27 b whose rear end surface defines a part of the intermediatefluid chamber (D) is made the same as the diameter (d2) of the plunger16. Therefore, the sum in volume of the electromagnetic section fluidchamber (B) and the intermediate fluid chamber (D) which are incommunication with each other through the clearance (C) between thecenter hole 13 a and the rod portion 26 a is kept invariable or constantregardless of the movement of the plunger 16. This advantageously makeit unnecessary to provide any communication passage which wouldotherwise become necessary for making the intermediate fluid chamber (D)communicate with the external and any communication groove (orcommunication hole, hereafter referred to simply as “communicationgroove) which would also otherwise become necessary for communicationbetween the rear end fluid chamber (A) and the electromagnetic sectionfluid chamber (B). Accordingly, the solenoid-operated valve in theembodiment can be simplified in construction. In particular, since theomission of such communication groove contributes to increasing themagnetic attraction force exerted on the plunger 16, thesolenoid-operated valve in the embodiment can be downsized.

[0030] When the plunger 16 and the spool 26 are moved toward the rightas viewed in FIG. 1, the volume of the electromagnetic section fluidchamber (B) decreases as the volume of the intermediate fluid chamber(D) increases, and the oil flows toward the right through the clearance(C) between the center hole 13 a and the rod portion 26 a. On thecontrary, when the plunger 16 and the spool 26 are moved toward the leftas viewed in FIG. 1, the oil flows toward the left through the clearance(C). That is, the direction in which the oil flows through the clearance(C) between the center hole 13 a and the rod portion 26 a upon movementof the plunger 16 and the spool 26 coincides with the moving directionof the rod portion 26 a. Thus, the viscosity of the oil flowing throughthe clearance (C) serves to aid or expedite the movement of the rodportion 26 a which is moved together with the spool 26, so that theresponsivety of the solenoid-operated valve can be enhanced.

[0031] In the foregoing embodiment, the annular groove 25 formed at themid portion of the small-diameter portion 22 b of the valve hole 22 orin-between the feedback chamber 24 and the intermediate fluid chamber(D) communicates with the drain port 23 c to expose the mid portion ofthe small-diameter land portion 27 b to the drain port 23 c. With thisarrangement, the oil including foreign matter such as iron powder whichoil leaks from the feedback chamber 24 through a clearance between thevalve hole 22 and the small-diameter land portion 27 b is dischargedfrom the annular groove 25 formed between the feedback chamber 24 andthe intermediate fluid chamber (D), to the drain port 23 c. Therefore,the foreign matter is prevented from entering the intermediate fluidchamber (D) thereby to increase the content of the foreign matter withinthe intermediate fluid chamber (D). Since the foreign matter hardlyenters the electromagnetic section fluid chamber (B) from the valvesection 20 through the intermediate fluid chamber (D), it hardly occurthat such foreign matter gets into the clearance between the inner bore11 a of the stator 11 and the plunger 16 to deteriorate the movement ofthe plunger or to bring the same into a lock. Or, the time can beextended for the plunger 16 to reach such undesirable state.

[0032] In the foregoing embodiment, the supply/drain passage 18 formaking the rear end fluid chamber (A) communicate with the external isconstituted by abutting the inner bottom surface of the cover 14 on therear end surface of the yoke 12 which has the annular groove 18 a, thechamfer 18 b and the cutouts 18 c, 18 d formed thereon. Since theprovision of the annular groove 18 a, the chamfer 18 b and the cutouts18 c, 18 d on the rear end surface of the yoke 12 can be carried outeasily by cutting or by metal forming, it can be attained to lower themanufacturing cost of the solenoid-operated valve. It is howeverapparent that the annular groove 18 a, the cutouts 18 c, 18 d and thelike which constitute the supply/drain passage 18 may be provided byforging or the like not on the rear end surface of the yoke 12 but onthe inner bottom surface or may be provided on both of the rear endsurface of the yoke 12 and the inner bottom surface of the cover 14.That is, since the supply/drain passage 18 can be constituted by forminga hollow groove at least one of the inner bottom surface of the cover 14and the rear end surface of the yoke 12 which are kept in abuttingengagement with each other, the provision of the supply/drain passage 18becomes easy, so that the manufacturing cost for the solenoid-operatedvalve can be reduced.

[0033] Further, in the foregoing embodiment, the supply/drain passage 18takes the form of a labyrinth, in which case the time is extended untilthe foreign matter such as minute iron powder flowing in the oil aroundthe solenoid-operated valve which oil comes in or out of the samethrough the supply/drain passage 18 reaches the rear end fluid chamber(A). Therefore, a substantial extension of time can be attained untilthe plunger 16 gets worse in movement or is brought into a lock. Thesesadvantages of the labyrinth supply/drain passage 18 can easily beappreciated in the following viewpoint. That is, through thesupply/drain passage 18, the oil around the solenoid-operated valve ischarged into the rear end fluid chamber (A) or discharged therefrom asthe same varies its volume with the movement of the plunger 16. Sincethe supply/drain passage 18 is formed to be a labyrinth, the foreignmatter such as minute iron powder flowing in the oil subsides ordeposits while moving back and forth in the labyrinth supply/drainpassage 18. As a result, the foreign matter is separated from the oilwhich comes into the rear end fluid chamber (A) and hence, can beprevented from entering the rear end fluid chamber (A) at the earlystage in use of the solenoid-operated valve.

[0034] Although the supply/drain passage 18 takes the form of alabyrinth, it is not limited to being the labyrinth and may rather be apassage simple in shape.

[0035] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, thepresent invention may be practiced otherwise than as specificallydescribed herein.

What is claimed is:
 1. A solenoid-operated valve comprising a stator having a yoke and a core arranged serially in axial alignment with each other; a plunger received in said stator to be slidably guided in an inner bore formed in at least one of said yoke and said core and urged by a spring in one direction; an electromagnetic coil for energizing said stator to move said plunger axially against said spring; a valve sleeve attached to an outer end portion of said core; a spool received slidably in a valve hole of said valve sleeve and connected to said plunger through a rod portion thereof passing through a center hole of said core to be movable together with said plunger; an electromagnetic section fluid chamber defined by a forward end of said plunger at the side of said valve sleeve and said inner bore of said core; an intermediate fluid chamber formed between said core and said valve sleeve and partly defined by an end surface of a land portion which is formed on said spool at the side of said core; and a clearance between said center hole and said rod portion for making said electromagnetic section fluid chamber and said intermediate fluid chamber communicate with each other; and a rear end fluid chamber formed between a rear end surface of said plunger and said inner bore of said yoke and variable in its volume with movement of said plunger; wherein the diameter of said plunger is chosen to be the same as that of said land portion and wherein said rear end fluid chamber communicates with the external of said solenoid-operated valve through a supply/drain passage.
 2. The solenoid-operated valve as set forth in claim 1, wherein the mid portion of said land portion communicates with a drain port formed in said valve sleeve.
 3. The solenoid-operated valve as set forth in claim 1, wherein said stator further includes a cover which covers the external surfaces of said yoke and said core thereby to magnetically connect the opposite end portions of said yoke and said core, and wherein said supply/drain passage is defined by a hollow groove formed on at least one of an inner bottom surface of said cover and the rear end surface of said yoke which are kept in abutting engagement with each other.
 4. The solenoid-operated valve as set forth in claim 3, wherein said supply/drain passage takes the form of a labyrinth. 